Disclosed herein are compounds that inhibit the activity of fatty acid amide hydrolase (FAAH), compositions that include the compounds, and methods of their use. Compounds disclosed herein as inhibitors of fatty acid amide hydrolase (FAAH) are useful in the treatment of diseases, disorders, or conditions that would benefit from the inhibition of fatty acid amide hydrolase and increases in endogenous fatty acid amides.
Fatty acid amide hydrolase (FAAH) is an enzyme that is abundantly expressed throughout the CNS (Freund et al. Physiol. Rev. 2003; 83:1017-1066) as well as in peripheral tissues, such as, for example, in the pancreas, brain, kidney, skeletal muscle, placenta, and liver (Giang, D. K. et al., Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 2238-2242; Cravatt et al. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 29, 10821-10826). FAAH hydrolyzes the fatty acid amide (FAA) family of endogenous signaling lipids. General classes of fatty acid amides include the N-acylethanolamides (NAEs) and fatty acid primary amides (FAPAs). Examples of NAEs include anandamide (AEA), palmitoylethanolamide (PEA) and oleoylethanolamide (OEA). An example of FAPAs includes 9-Z-octadecenamide or oleamide. (McKinney M K and Cravatt B F. 2005. Annu Rev Biochem 74:411-32). Another class of fatty acid amide family of endogenous signaling lipids is N-acyl taurines that have also been shown to be elevated upon FAAH deletion or inhibition and appear to act on transient receptor potential (TRP) family of calcium channels, although the functional consequences are not yet clear (Saghatelian A, et al. Biochemistry. 2004, 43:14332-9, Saghatelian A, et al. Biochemistry, 2006, 45:9007-9015). In addition to fatty acid amides, FAAH can also hydrolyze certain fatty acid esters, such as, for example, 2-arachidonylglycerol (2-AG) another endocannabinoid (Mechoulam et al. Biochem. Pharmacol. 1995; 50:83-90; Stella et al. Nature, 1997; 388:773-778; Suguria et al. Biochem. Biophys. Res. Commun. 1995; 215:89-97).
Inhibition of FAAH is expected to lead to an increase in the level of anandamide and other fatty acid amides. This increase in fatty acid amides leads to an increase in the noiceptive threshold. Thus, inhibitors of FAAH are useful in the treatment of pain (Cravatt, B F; Lichtman, A H Current Opinion in Chemical Biology 2003, 7, 469-475). Such inhibitors are useful in the treatment of other disorders that can be treated using fatty acid amides or modulators of cannabinoid receptors, such as, for example, anxiety, sleep disorder, Alzheimer disease, and Parkinson's disease, eating disorders, metabolic disorders, cardiovascular disorders, and inflammation (Simon et al Archives of Gen. Psychiatry, 2006, 63, 824-830. Kunos, G et al. Pharmacol Rev 2006, 58,389-462). In some embodiments, FAAH inhibitor compounds may be peripherally restricted and may not substantially affect neural disorders, such as, for example, depression and anxiety. Finally, agonism of cannabinoid receptors has also been shown to reduce the progression of atherosclerosis in animal models (see Steffens et al. Nature, 2005, 434, 782-786; and Steffens et al., Curr Opin. Lipid., 2006, 17, 519-526). Thus, increasing the level of endogenous cannabinergic fatty acid amides (e.g., anandamide) is expected to effectively treat or reduce the risk of developing atherosclerosis.
Inhibition of FAAH also leads to elevation of palmitoylethanolamide which is thought to work, in part, through activation of the peroxisome proliferator-activated receptor α (PPAR-α) to regulate multiple pathways including, for example, pain perception in neuropathic and inflammatory conditions such as convulsions, neurotoxicity, spacticity and to reduce inflammation, for example, in atopic eczema and arthritis (LoVerme J et al. The nuclear receptor peroxisome proliferator-activated receptor-alpha mediates the anti-inflammatory actions of palmitoylethanolamide. Mol Pharmacol 2005, 67, 15-19; LoVerme J et al The search for the palmitoylethanolamide receptor. Life Sci 2005, 77: 1685-1698. Lambert D M et al. The palmitoylethanolamide family: a new class of anti-inflammatory agents? Curr Med Chem 2002, 9: 663-674; Eberlein B, et al. Adjuvant treatment of atopic eczema: assessment of an emollient containing N-palmitoylethanolamine (ATOPA study). J Eur Acad Dermatol Venereol. 2008, 22:73-82. Re G, et al. Palmitoylethanolamide, endocannabinoids and related cannabimimetic compounds in protection against tissue inflammation and pain: potential use in companion animals. Vet J. 2007 173:21-30.). Thus, inhibition of FAAH is useful for the treatment of various pain and inflammatory conditions, such as osteoarthritis, rheumatoid arthritis, diabetic neuropathy, postherpetic neuralgia, skeletomuscular pain, and fibromyalgia.
It is also thought that certain fatty acid amides, such as, for example, OEA, act through the peroxisome proliferator-activated receptor α (PPAR-α) to regulate diverse physiological processes, including, e.g., feeding and lipolysis. Consistent with this, human adipose tissue has been shown to bind and metabolize endocannabinoids such as anandamide and 2-arachidonylglycerol (see Spoto et al., Biochimie 2006, 88, 1889-1897; and Matias et al., J. Clin. Endocrin. & Met., 2006, 91, 3171-3180). Thus, inhibiting FAAH activity in vivo leads to reduced body fat, body weight, caloric intake, and liver triglyceride levels. However, unlike other anti-lipidemic agents that act through PPAR-α, e.g., fibrates, FAAH inhibitors do not cause adverse side effects such as rash, fatigue, headache, erectile dysfunction, and, more rarely, anemia, leukopenia, angioedema, and hepatitis (see, e.g., Muscari et al. Cardiology, 2002, 97: 115-121).
Many fatty acid amides are produced on demand and rapidly degraded by FAAH. As a result, hydrolysis by FAAH is considered to be one of the essential steps in the regulation of fatty acid amide levels in the central nervous system as well as in peripheral tissues and fluids. The broad distribution of FAAH combined with the broad array of biological effects of fatty acid amides (both endocannabinoid and non-endocannabinoid mechanisms) suggests that inhibition of FAAH leads to altered levels of fatty acid amides in many tissues and fluids and may be useful to treat many different conditions. FAAH inhibitors increase the levels of endogenous fatty acid amides. FAAH inhibitors block the degradation of endocannabinoids and increase the tissue levels of these endogenous substances. FAAH inhibitors can be used in this respect in the prevention and treatment of pathologies in which endogenous cannabinoids and or any other substrates metabolized by the FAAH enzyme are involved.
The various fatty acid ethanolamides have important and diverse physiological functions. As a result, inhibitor molecules that selectively inhibit FAAH enzymatic activity would allow a corresponding selective modulation of the cellular and extra-cellular concentrations of a FAAH substrate. FAAH inhibitors that are biologically compatible could be effective pharmaceutical compounds when formulated as therapeutic agents for any clinical indication where FAAH enzymatic inhibition is desired. In some embodiments, FAAH activity in peripheral tissues can be preferentially inhibited. In some embodiments, FAAH inhibitors that do substantially cross the blood-brain-barrier can be used to preferentially inhibit FAAH activity in peripheral tissues. In some embodiments, FAAH inhibitors that preferentially inhibit FAAH activity in peripheral tissues can minimize the effects of FAAH inhibition in the central nervous system. In some embodiments, it is preferred to inhibit FAAH activity in peripheral tissues and minimize FAAH inhibition in the central nervous system.